Seams, particularly in parachutes, are made by folding edges of the fabric material and stitching the folded seam with two or four needle sewing machines. A typical personnel parachute has a full canopy about 30 feet in diameter which is basically an assembly of 28 gores, each gore about 14 feet long, radiating outward from the center of the canopy. The center of the canopy, called the vent, joins the top of the 28 gores forming the parachute. The bottom of the parachute canopy is called the skirt, along which the ends of the gores are sewn into a skirt hem. Adjacent sections of the 28 gores are held together by what is called main seams. These main seams are four layer fell seams which are generally about one inch wide and have four layers of folded material comprised of one inner and one outer fold layer from each gore edge. Each gore's inner fold is one inch wide and lies between the other gore's inner and outer folds.
The canopy assembly is formed by joining the first gore to a second gore by taking one edge from each gore and folding and sewing the edges into a four layer fell seam main seam. Thus each additional gore is joined to the canopy by a main seam.
Each gore is generally made of several three or four foot long selvages sewn together by one half inch wide four layer fell seams called cross seams which run circumferentially around the canopy. These cross seams are generally not hard for the operators to sew because they are narrower, only 1/2 inch in width, than main seams and because they are folded parallel to the warp (or fill) of the woven nylon fabric. However, main seams between adjacent gores are folded at about 45 degree bias angle to the warp (or fill) which is a complicating aspect of main seam sewing. The susceptibility of fabric to folding is inversely proportional to the angle between the fold and the warp (or fill). Parachutes generally are manufactured from 1.1 ounce or so ripstop nylon fabric that is approximately 0.007 (7 thousandths of an inch) thick, which will form four layer fell seams approximately 0.032 (32 thousandths) thick comprised of four layers of material plus thread.
Since before World War II, parachute canopy main seams have been formed by a simple sheet metal folder about 5 inches long which is used to make one inch wide four layer fell seam main seams. The presently used sheet metal folder has two 180 degree contours which interfold the two edges of the gores. The present method of manufacturing the main seam requires a great deal of skilled sewing operator labor in terms of manual dexterity and job experience since the operator must prefold each of the two edges exactly one inch for proper inner-fold width and simultaneously rotate each prefolded edge 90 degrees and simultaneously maintain exactly the right amount of tension in each of the two gore fabrics against the pull of the sewing machine feed. The material is folded over and fed through the sheet metal folder into the bite of a two or four needle sewing machine and is pulled along by the action of the reciprocating feed dog and a clamping foot of the sewing machine. The existing sheetmetal folder plays no role in the proper tensioning or edge alignment of the two edges. Thus at present it takes a great deal of skilled labor to continuously fold and sew parachute main seams, and other fabric fell seams wider than 1/2 inch.
It is therefore one objective of the present invention to provide an automatic feeding and folding die for delivering folded seams to a sewing machine.
Still another objective of the present invention is to provide an automatic feeding and folding die which pre-folds the seam prior to its entry into the sewing machine.
Yet another objective of the present invention is to provide an automatic feeding and folding die for the continuous fabric feeding and fabric edge folding of a canopy fabric edge and an adjacent gore fabric edge into a four layer fell seam main seam.